1. FIELD OF THE INVENTION
The present invention relates to a gasoline direct-injection engine including a cavity provided in a top portion of a piston and forming a portion of a combustion chamber defined between a cylinder head and the top portion of the piston, a fuel injection valve mounted in the cylinder head for injecting gasoline directly into the combustion chamber, and a spark plug also mounted in the cylinder head to face the combustion chamber.
2. DESCRIPTION OF THE RELATED ART
A gasoline direct-injection engine shown in FIG. 14 and a gasoline direct-injection engine shown in FIG. 15 are conventionally known. In the gasoline direct-injection engine shown in FIG. 14, a fuel injecting valve 22 with its axis inclined with respect to the axis of a piston 13.sub.4 is mounted in a cylinder head 14 to be able to inject gasoline into a cavity 21.sub.1 provided in a top portion of the piston 13.sub.4, and a spark plug 23 is mounted in the cylinder head 14 in such a manner that its front end faces into a flow of gasoline injected from the fuel injection valve 22. Before an air-gasoline mixture is uniformized within the combustion chamber 151, the gasoline can be burned by igniting the gasoline injected from the fuel injection valve 22, thereby enabling a lean laminar burn. In the gasoline direct-injection engine shown in FIG. 15, a fuel injection valve 22 facing a side of a combustion chamber 15.sub.2 is mounted in a cylinder head 14 to have its axis inclined with respect to the axis of a piston 13.sub.5, and a spark plug 23 is mounted in the cylinder head 14 to face a substantially central portion of a ceiling surface of the combustion chamber 15.sub.2. A cavity 21.sub.2 is provided in a top portion of the piston 13.sub.5 and has a shape permitting gasoline to be guided to a portion around the spark plug 23 by the penetration of gasoline injected from the fuel injection valve 22 and by a swirl flow generated within the combustion chamber 15.sub.2. Thus, it is possible to achieve a lean burn by injecting the gasoline from the fuel injection valve 22 in the later portion of a compression stroke.
In an engine designed to ignite gasoline, which is being injected from the fuel injection valve 22, by a spark ignition provided by the spark plug 23 as in the gasoline direct-injection engine shown in FIG. 14, however, the vaporization of the gasoline and the mixing of the gasoline with air in the combustion chamber 15.sub.1 are insufficient. Therefore, it is difficult to provide a stable ignition by a usual igniting system and hence, a multiplex ignition is required, and problems arise such as a complication of the igniting system, an increase in cost, and wear in electrodes of the spark plug. In addition, due to an insufficient mixing of the gasoline and air, a smoke is liable to be produced, and the amount of unburned gasoline discharged is relatively large, thereby bringing about a deterioration in the nature of an exhaust gas. Further, the burning period is prolonged to make it difficult to improve the fuel consumption, and a stable lean laminar burn cannot be performed in wide ranges of rotational speed and load of the engine.
In the gasoline direct-injection engine shown in FIG. 15, the combustible air-gasoline mixture is collected to a portion near the spark plug 23 by virtue of the shape of the cavity 21.sub.2 and by the swirl flow within the combustion chamber 15.sub.2, so that a laminar burn is carried out. Therefore, the timing of injection by the fuel injection valve 22 is limited and for this reason, it is impossible to perform a stable lean laminar burn in wide ranges of rotational speed and load of the engine. When an early injection is carried out by the fuel injection valve 22 in a full-load operation or the like in order to provide a uniform air-gasoline mixture, the injected gasoline is liable to be deposited to a cylinder wall surface on the side opposite to the fuel injection valve, and there is a possibility that a deterioration in engine lubricating oil, a degradation in durability of an engine body and a deterioration in the nature of an exhaust gas may be brought about.